Petroleum is currently estimated to account for over 35% of the world's total commercial primary energy consumption. Coal ranks second with 23% and natural gas third with 21%. The use of liquid hydrocarbon fuels on an enormous scale for transportation has led to the depletion of readily accessible petroleum reserves in politically stable regions and this, in turn, has focused attention, economically, technically and politically on the development of alternative sources of liquid fuels.
One established route to the production of hydrocarbon liquids is the gasification of carbonaceous materials followed by the conversion of the produced synthesis gas to form liquids by processes such as Fischer-Tropsch and its variants. In this way, solid fuels such as coal and coke may be converted to liquids. Coal gasification is well-established, being used in many electric power plants. Gasification can proceed from just about any organic material, including biomass, paper, plastic and rubber waste. Most importantly, in a time of unpredictable variations in the prices of electricity and fuels, gasification systems can provide a capability to operate on low-cost, widely-available coal reserves. Gasification may be one of the best ways to produce clean liquid fuels and chemical intermediates from coal as well as clean-burning hydrogen which also can be used to fuel power-generating turbines or used in the manufacture of a wide range of commercial products.
Four basic types of gasifiers are currently available for commercial use: counter-current bed, co-current bed, fluidized bed and entrained flow. In the counter-current fixed bed (“up draft”) gasifier the gasification agent (steam, oxygen and/or air) flows in counter-current configuration through a descending bed of the carbon-containing fuel with the ash removed dry or as a slag. The co-current bed gasifier is similar to the counter-current type, but the gasification agent gas flows downwards in the same direction as the fuel. In the fluidized bed reactor, the fuel is fluidized in the gasification agent. In the entrained flow gasifier a dry pulverized solid, an atomized liquid fuel or a fuel slurry is gasified with oxygen or air in co-current flow and the gasification reactions take place in a dense cloud of very fine particles. Most coals are suitable for this type of gasifier because of the high operating temperatures and because the good contact achieved between the coal particles and the gasifying agent. Entrained flow gasifiers have been demonstrated as highly effective units for the gasification of coal and other carbonaceous fuels such as coke (coal derived), deasphalter bottoms and petroleum coke.
Biomass gasification is expected to play a significant role in a renewable energy economy, because biomass production removes CO2 from the atmosphere and the net effect of processing the biomass has a net lower CO2 generation as compared to fossil fuels. While other biofuel technologies such as biogas and biodiesel are also beneficial fuel sources for reducing carbon emissions, gasification runs on a wider variety of input materials, can be used to produce a wider variety of output fuels, and is an extremely efficient method of extracting energy from biomass. Biomass gasification is therefore one of the most technically and economically viable energy possibilities for a carbon emission constrained economy.
In power generation, the gasification of biomass exhibits the following main advantages over conventional combustion technologies:                1) Higher electrical efficiencies in the range of 22% to 37% can be achieved in gasification compared to biomass combustion technologies utilizing steam generation and steam turbines (15% to 18%).        2) There is less CO2 emissions associated with the gasification than with the combustion of similar fuels.        
The integration of biomass gasification in existing large coal-fired power stations is being investigated in different countries. Integration is currently more practical than stand-alone biomass gasification due to the greater flexibility in response to annual and seasonal fluctuations in biomass availability and the lower investment costs for the biomass gasification unit.
Co-gasification or co-firing of coal and biomass can be carried out via a number of processes:                Co-feeding biomass and coal to the gasifier as a mixture.        Co-feeding biomass and coal to the gasifier using separate gasifier feed systems.        Pyrolizing the biomass followed by co-feeding pyrolysis char and coal to the gasifier.        Gasifying the biomass and coal in separate gasifiers followed by a combined fuel gas clean-up.        
There are, however, some drawbacks of co-firing/co-gasification of biomass and coal. Biomass has a high moisture content that results in low calorific value of the fuel, low bulk density compared to coal, low ash melting point, chemical composition with potentially high chlorine content, as well as hydrophilic and non-friable characteristics. Additionally, co-firing/co-gasification faces the possibility of decreases in overall efficiency, deposit formation (slagging and fouling), agglomeration, corrosion and/or erosion, and ash utilization. The importance of the problems depends upon the biomass/coal ratios and the quality of the biomass used as a feedstock; especially in direct co-firing systems without dedicated biomass infrastructure. During co-gasification, ash-forming species from biomass may either leave the process as bottom ash, or become released as fly ashes and flue dust. The fate of these species is dependent on their physical characteristics, the chemistry, the equipment design and the combustion conditions. These problems are further complicated by the presence of alkali metals (K, Na), alkaline earth metals (Ca, Mg), silicon, chlorine and sulphur in the ashes and can lead to reduced efficiency, capacity and availability of the facilities and therefore increased power cost.
Reburning, a combustion modification technology originally introduced by the John Zink™ Company, provides one way to integrate biomass and coal power generation minimizing the problems described above. In the reburning process, the biomass is pre-gasified and the resulting gas is used as reburn fuel in a coal-fired boiler that uses the fuel as a reducing agent to remove NOx from the combustion products. This process has the advantage of keeping undesired components, such as alkaline, chlorine, and heavy metal compounds commonly associated with biomass, away from the coal-fired boiler and, in addition, the environmental aspects of air pollution, as well as operational and economical problems, such as slagging, fouling, corrosion, and contamination of ash, are avoided. During the gasification of biomass, the solid feedstock is separated into fuel gas and a solid residue. The aim of the separate gasification of biomass is to bond the problem-causing components into the solid pyrolysis residue in order to avoid operational problems within coal-fired boilers.
With entrained flow gasifiers operating with coal-biomass mixture fuels, one problem is the delivery of the feedstock mixture of carbonaceous solids and biomass to the gasifier. Different types of entrained flow gasifiers feeding solid coal or coal-water slurries with wood chips have been reported to encounter feedstock delivery as one of the hurdles to continuous running. Failure of slurry pumps and the clogging of lock hoppers have been observed. It is therefore desirable to develop a way of feeding biomass to entrained flow gasifiers which does not suffer from these disadvantages.